1. Field of the Invention
The invention relates to a video signal-processing device comprising an arrangement for detecting film scratches. The invention also relates to the arrangement for detecting film scratches as such, and to a method of detecting scratches.
2. Description of the Related Art
Older films, particularly those which are frequently used, often have vertical scratches which extend continuously through longer parts of the film. These scratches may have already been produced in the film camera or during the copying process, or also by frequent playback of the film.
When such film material is displayed, the scratches occur as white or black horizontal stripes in positive or negative film material, respectively. A method and arrangement for detecting and concealing errors in a video signal caused by vertical film scratches is known from German offenlegungsschrift DE 44 32 787.0, corresponding to U.S Pat. No. 5,589,887, in which an error signal is derived from disturbed spots produced by disturbed pixels and used in the form of a control signal for concealing the errors in the video signal. To be able to safely distinguish scratches from other, vertically extending picture details such as, for example, flag poles, the error signal is only generated when an adjustable, locally limited scratch width in the horizontal direction is not exceeded and when each scratch essentially extends vertically, and when the scratch length corresponds approximately to the frame height. The disturbed spots in the video signal are subsequently replaced by the corresponding low-pass-filtered video signal with the aid of the error signal, so that the pixels of the scratch assume the brightness and color of the pixels in the close surroundings of the scratch and very strong contrast differences can thus be avoided.
It has been described in the German Offenlegungsschrift DE 44 32 787.0 that, to evaluate the length expansion of a scratch, superjacent pixels of a scratch which does not extend throughout the height of an image are expanded throughout the image, height by means of line sorting and vertical low-pass filtering with the aid of a median filter, as from a certain minimum expansion of the scratch. The minimum value from which a scratch is expanded can only be indirectly predetermined via the number of taps on the median filter; the length expansion of slant scratches cannot be performed with this arrangement.
However, in the detection of scratches, the determination of the scratch length was a decisive feature of distinction, because a decision which was only based on other scratch features, such as the width of the scratch and contrast distinctions caused by the scratch, could also be caused by other image details which should certainly not be suppressed. Moreover, in the known circuit arrangement, the impurity concealment signal used for concealing the scratch covers the area of a video image which a rectangle in the video image circumscribing the scratch would occupy. The larger the inclination of a scratch, the larger the area which should actually not be concealed.
It is therefore an object of the invention to provide a method and a circuit arrangement for processing video signals in which scratches in the video signal, particularly slant scratches can be reliably distinguished from other, scratch-like image structures. It is another object of the invention to generate a scratch mask, or an impurity concealment signal derived therefrom, which better corresponds to the shape of a slant scratch.
According to the invention, in an arrangement for detecting scratches, this object is solved in that the arrangement for detecting scratches is intended to combine a given number of series-arranged pixels of a video image to form a group, and to determine a division of the pixels in accordance with given criteria for each group of pixels and, based on this division, to classify the pixel groups in scratch sections and non-scratch sections.
The invention is based on the recognition that for an evaluation of the accumulation of pixels with corresponding properties, which may also be expected in scratches (width, contrast, . . . ) within a given image column with slant scratches, or scratches with interruptions, the threshold value used for detection had to be decreased to such an extent that the confusion of the remaining pixels of a scratch with pixels having similar properties but constituting actual objects in the image would become more and more probable. The assessment criterion proposed in the invention, namely that not only each individual pixel must fulfil a given criterion but that a predetermined number of pixels must have a given minimum division of properties, reduces the probability that accidentally distributed pixels within this group also have this property. In this way, a scratch is no longer examined as a whole, for example, whether a sufficient number of pixels having corresponding properties can be combined to form a scratch, but that accumulation and alignment of image details having given structural properties are evaluated as the actual presence of a scratch. Since image objects with corresponding structural properties occur considerably less frequently than image objects composed of an equal number of accidentally distributed individual pixels with the corresponding properties, a more reliable answer as to whether a scratch is actually present is obtained when the structural properties of presumable scratches are examined.
To this end, an embodiment of the invention is characterized in that the number of pixel groups, classified as scratch sections of an image area of the video image in which a scratch is suspected, is determined, and this number is compared with a first threshold value, the suspected scratch being signalized as a recognized scratch only when the first threshold value is exceeded.
The invention is based on the recognition that a scratch must be composed of a given minimum number of scratch sections so as to be accepted as a scratch. If the number of detected scratch sections is too low, image details are mostly concerned which comply with the criteria for scratch sections but are actually partial structures of an image object. Since the accumulation of such image structures occurs less frequently than the accidental occurrence of pixels, the decision about the number of scratch sections is much more reliable.
Since for a corresponding accumulation of scratch sections an actual scratch is concerned, sections which do not have the accumulation of a continuous scratch may be accepted as scratches, i.e., also partially interrupted scratches are accepted. This is particularly advantageous because, for example, a black scratch which extends through a dark image area is present but is no longer visible. It is true that this scratch need not be concealed, but it reduces the measurable overall length of a scratch when it is examined in the conventional manner and would thus prevent detection of the scratch.
In an advantageous embodiment of the invention, the decision threshold on which part of a scratch should at least be visible to be still accepted as a scratch can be adapted to the relevant image contents by changing the threshold value.
Preferably, the pixel groups classified as scratch sections may be used for forming a scratch mask by means of which the scratch can be concealed. This has the advantage that a mask formed in that way can be adapted much more accurately to the shape of a scratch, as compared with the situation where the mask would extend beyond the image section circumscribed as a rectangle by the scratch.
A very simple embodiment of the invention is obtained when the pixels occurring in a given number of vertically subjacent lines at the same horizontal position are combined to form pixel groups. In this way, each video image is divided into strips and only the directly subjacent pixels of a strip are considered. This has the advantage that scratch sections can be detected without much effort.
In a special embodiment of the invention, the high-pass-filtered pixels are compared, for the purpose of classification of scratch sections, with at least a contrast threshold value, the sum of the pixels of a pixel group exceeding a given contrast threshold value are determined and, in so far as this sum exceeds a further threshold value, this pixel group is classified as a scratch section.
Particularly high-pass-filtered signals essentially contain only signal parts of pulse-shaped image objects. If these pulse-shaped image objects also exceed a given contrast difference, then a pre-selection has already been made among the image signals qualifying as scratches anyway. However, only when a given minimum number of pixels also has the same contrast difference, it can be assumed for this pixel group that it has a structure which is similar to that of segments or sections of scratches.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.